The subject matter disclosed herein relates to thin-film materials that can be utilized in reflection-controllable electrochromic windows (i.e., light-control glass) for buildings, vehicles, aircraft and watercraft. More particularly, the subject matter disclosed herein relates to reflection-controllable electrochromic thin-film materials comprising a metal-chalcogen compound in which one or more of the metals comprise Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Sb, or Bi, or combinations thereof and one or more of the chalcogens comprise O, S, Se, or Te, or combinations thereof.
Windows and other openings are generally the place where the most heat enters and escapes buildings. For example, during the winter about 48% of the heat produced by a heating system of a building escapes through windows of the building. During the summer, the proportion of heat that enters an air-conditioned room through the windows can reach about 71%. A tremendous energy savings can, therefore, be realized by effectively controlling light and heat entering and escaping through windows. Light-control glass has been developed to control the bi-directional flow of light and heat through a window.
There are several ways that light is controlled by light-control glass. One way is to form an electrochromic material on the glass in which the transmissivity of the electrochromic material reversibly changes upon application of a current or a voltage. Another way is to form a thermochromic material on the glass in which the transmissivity of the thermochromic material changes with temperature. Yet another way is to use a gasochromic material that changes its transmissivity by controlling the atmosphere gas. Of these, electrochromic-based light-control glass has been researched in which a tungsten-oxide thin film is used for the light-control layer. Some commercial products based on this type of electrochromic light-control glass have already appeared.
Conventional electrochromic-based light-control glass, including tungsten-oxide-based versions, all control light by absorbing the light using a light-control layer. A drawback with absorbing the light is that heat is produced and radiated into a room when the light-control layer absorbs light, thereby diminishing the energy-saving effect of the conventional electrochromic light-control glass. To eliminate this drawback, another approach of reflecting light rather than absorbing light has been considered. Accordingly, a material capable of reversibly switching between a mirror state and a transparent state would be useful.
For a long time, such a material capable of switching between a mirror state and a transparent state was not found, but in 1996 a group in the Netherlands discovered a hydride of a rare earth, such as yttrium or lanthanum, switches between a mirror state and a transparent state under the influence of hydrogen. Such a material is conventionally referred to as a “switchable mirror”. See, for example, J. N. Huiberts et al., Nature, 380, 1996, 231. The rare-earth hydrides undergo a large change in transmissivity, and have excellent light-control mirror characteristics. Nevertheless, because a rare-earth element is used in the material, there are problems in terms of resources and cost when rare-earth-hydride-based switchable mirrors are used for window coatings and other applications.
Additionally, conventional metal-hydride-based mirrors suffer from poor cycle life due to the reactive nature of the metal film, which is readily attacked by air or water. Notably, water is one component of the electrolyte in electrochromic hydride mirrors, and may be produced during removal of hydrogen from the mirror film in both electrochromic and gasochromic devices. The life-cycle degradation is conventionally inhibited by using additional barrier layers for protecting the active materials and by sealing devices for preventing access of environmental air and water. The former approach of adding barrier layers is difficult to achieve and may not be effective after long periods of use. The latter approach of sealing does not address the problem of internal sources of water or oxygen.
More recently, U.S. Pat. No. 6,647,166 B2 to T. J. Richardson discloses alloys of magnesium and transitional-metals that can be used as switchable-mirror materials, thereby significantly reducing the cost of materials for electrochromic-based light-control glass.
U.S. Pat. No. 7,042,615 B2 to T. J. Richardson discloses the use of a semi-metal, such as antimony, as a switchable-mirror thin-film material based on alloying and de-alloying of the semi-metal with lithium: Sb+3Li++3e−=Li3Sb. Such elements-based thin-film materials, however, suffer from several severe drawbacks for the following reasons. (1) The electrochromic reaction does not take place until very low potential (about 0.7 V vs. Li/Li+), thus preventing the use of known transparent conducting oxides, such as ITO and FTO, as the transparent electrodes in an electrochromic device. (2) The reaction involves a very large percentage of volume expansion/contraction upon full lithiation/delithiation (about 136% for pure Sb), causing problems of pulverization and de-lamination of the reflective layers. (3) Low-resistivity metals (e.g., Ni and Co) need to be added to the reflective layers to reduce the percentage of volume change and increase the conductivity of the matrix, but the added metal absorbs a significant portion of the incident light and lowers the maximum transmission of an electrochromic window. (4) The electrochromic reaction involving the extruding and re-admission of the non-active metal is a non-topotactic and, thus, poorly reversible reaction. (5) A direct-current voltage higher than about 4 V is needed to drive an electrochromic device employing such an electrochromic material, which accelerates the degradation of an electrochromic device during long-term use or cycling due to side reactions associated with high driving-voltages.
Therefore, there is an urgent need in the electrochromic fields to discover reflection-controllable electrochromic thin-film materials based on intercalation compounds with intrinsic open structures for highly reversible electro-optical switching at a potential compatible with known transparent conducting oxide electrodes.
In U.S. Pat. No. 7,042,615 B2, T. J. Richardson discloses the use of transition-metal dichalcogenides (including TiS2, NbSe2, and NbTe2) as switchable-mirror thin-film materials, based on the contemplation that these semiconducting solids become metallic upon lithium insertion or intercalation (CATHODIC electrochromism). These chalcogen-rich transition-metal chalcogenides, however, show too narrow electro-optical switching ranges or too low coloration efficiencies, if any, to be useful in practical electrochromic devices.